According to a method of forming a film of a gallium nitride crystal layer by vapor phase process, it is difficult to bent dislocations accompanied with crystal growth. It is thus difficult to reduce the dislocations, without forced reduction of the dislocations such as ELO on the surface of seed crystal or growth using a PSS sapphire.
On the other hand, according to Na flux method, dislocations in the seed crystal substrate are succeeded by gallium nitride crystal film formed thereon. However, as the crystal growth, edge dislocations are bent and concentrated to reduce the dislocation density. It is thus possible to improve the crystal quality of the upper gallium nitride crystal than that of the seed crystal substrate, without the formed reduction of the dislocations.
To the date, it has been proposed and developed many kinds of techniques of forming a film of gallium nitride by flux method or the like on a c-plane (polarized plane) of a seed crystal substrate, and it is obtained gallium nitride crystal having a low dislocation density stably. However, it has not been developed a method of forming a film of gallium nitride on a non-polar seed crystal substrate, and its practical application is strongly demanded.
It is proposed a method of forming a self-supporting substrate by forming many stripe-shaped seed crystal layers on a seed crystal substrate to reduce defects in a gallium nitride crystal layer formed thereon and by spontaneously separating the gallium nitride crystal layer (Patent document 1 (Japanese Patent publication No. 2010-163288A): Patent document 2 (Japanese Patent Publication No. 2009-018975A): Patent document 3 (Japanese Patent Publication No. 2009-018972A): Patent document 4 (WO 2011/004904 A1)). It is, however, to form a film of gallium nitride on a polarized plane (c-plane) of the seed crystal substrate and to utilize ELO (Epitaxial lateral growth).
On the other hand, in the case that gallium nitride crystal is formed on the non-polar seed crystal layer, as techniques of reducing defects characteristic to the non-polar one, it may be listed Patent document 5 (Japanese Patent No. 4908467B), Patent document 6 (Japanese Patent No. 4793132B), Patent document 7 (Japanese Patent No. 4935700B), Patent document 8 (Japanese Patent Publication No. 2012-037153A), Patent document 9 (WO 2012/121154 A), Non-patent document 1 (Appli. Phys. Lett. 88., 061908), Non-patent document 2 (Appl. Phys. Lett. 98., 121916) or the like.